1Field of the Invention
The present invention relates to a plastic optical element configured to collect and scan beam emitted from a light source, a mold or forming the plastic optical element, a light scanning device including the plastic optical element and configured to scan beam emitted from at least one light source section and form an image based on image information, and an image forming apparatus having the light scanning device, which is, for example, an optical instrument such as a video camera or the like, a copying machine or forming a toner image trough an image forming process of an electrophotographic method, a facsimile device, a printer or a complex machine thereof, or the like.
2. Description of Related Art
Conventionally, there is known a light canning device installed in a multi-colored image forming apparatus configured to form images based on image information on photoconductor bodies by guiding beams emitted from a plurality of laser sources to the photoconductor bodies through a deflection device and an imaging device.
In recent years, to accomplish speeding up and high-quality image of the multi-colored image forming apparatus, an image forming apparatus such as a digital copying machine or laser printer or the like which is configured to arrange four photoconductor drums in a feeding direction of papers, expose images on the photoconductor drums by beams simultaneously, develop the images by a developer using different colors, yellow, magenta, cyan, and black, and transfer the images to the paper sequentially to overlap the images or forming a colored image has been put to practical use.
When light scanning is performed by use of the image forming apparatus, a plurality of light scanning devices are used. However, a large space is required for arrangement of the plurality of scanning devices. This results in a large-scaled image forming apparatus.
Therefore, a light scanning device including a single deflector for inputting and scanning a plurality of beams and imaging lenses which are stacked is known (or reference, Japanese Patent Laid-Open No. H 4-127115).
Known is also a light scanning device which includes a single deflector for inputting and scanning a plurality of beams, and imaging devices each of which is disposed to face a corresponding photoconductor for imaging an image on the photoconductor, and is configured to dispose optical elements constituting the imaging devices in a integrally stacked state in a secondary scanning direction perpendicular to a scanning direction of beam (for reference, Japanese Patent Laid-Open No. H 10-148777).
In this way, by stacking the optical elements in the secondary scanning direction, a thickness of a polygonal mirror or stacked polygonal mirrors in the deflector or deflection section can be reduced. Because one polygonal mirror is sufficient to scan the beams, it is possible to reduce a load on a motor for rotating the polygonal mirror and accomplish a compact light scanning device.
On the other hand, the following two proposals to demands for low cost of an optical element(s) or lens installed in the light scanning device have been made.
(1) Change From Material to Plastic Material                It is possible to form a transfer surface having a special mirrored shape and an outer shape of a wall biased in a longitudinal direction or scanning direction by an inexpensive material and low cost due to mass production.        
(2) Introduction of an Aspheric Shape                It is possible to reduce the number of parts by introduction of a longitudinal asymmetric aspheric shape or the like to the mirrored shape.        
In addition, the three proposals to demands for high accuracy of the optical element have been made.
(1) Reduction in Refractive-index Distribution                By disposing an optical alas at a position other than a center of an outer shape of a transfer su as proposed by the same inventors of the present application as in the preceding application, it is possible to reduce deterioration of an optical character due to an uneven refractive index (referred to as refractive-index distribution) existing inside the optical element. In this case, a characteristic that a primary factor (inclination) in a function of the refractive-index distribution does not affect the optical characteristic is utilized.        
(2) Introduction of an Aspheric Shape                It is possible to reduce aberration in the optical element by introduction of the aspheric shape (longitudinal asymmetric aspheric shape) to the mirrored shape. In other words, the optical characteristic is improved.        
(3) High accuracy of a Mirrored Surface                In a conventional plastic forming, it is desirable to uniform a rosin temperature and a resin pressure in a cavity of a mold in a process for cooling and solidifying a melted resinous material in the cavity in order to form a plastic molded article into a desired shape.        
For example, the transfer surface has low farm accuracy for the reason that when the lens includes a shape having a biased wall, a cooling velocity of the resin by a difference of thickness of the optical element or lens is different every each location on the optical element to generate a different amount of volume contraction.
Therefore, by providing a concave non transfer surface formed by incomplete transfer in a position other than the transfer surface, a structure configured to reduce an inner pressure or internal strain in the resin, accomplish similar production cost to that of a molded article having a thin wall, and acquire a high accuracy plastic molded article is, also, known (see Japanese Patent Laid-Open No. 2000-84945).
As a concrete method for forming a concave non transfer slue on a portion of a surface other than the transfer surface, a method for forming the non-transfer surface by defining forcibly a gap between the resin to form the optical element and one cavity forming piece has been known (see Japanese Patent Laid-Open No. H 11-28745 and Japanese Patent Laid-Open No. 2000-141413).
The method includes the steps of providing slidably a part of the cavity forming piece for forming a s Glee including the non-transfer surface, preparing a pair of molds including at least one cavity armed by the cavity piece and the transfer surface, heating the mold at a temperature lower than a softening temperature of the resin and maintaining at that temperature, filling the cavity with the melted resin heated at a temperature higher than the softening temperature to perform injection mold, then allowing a resin pressure to occur in the transfer surface to closely fit the resin with the transfer surface, and thereafter, sliding the slidable cavity forming piece to separate from the resin when the resin is cooled to a temperature lower than the softening temperature.
However, with demands to achieve further high quality image in recent years, there is a significant problem on a process that the transfer surface adjacent the aura defining the gap forcibly between the resin and the cavity forming piece or the surface including the non-transfer surface has low form accuracy.
The non-transfer surface is difficult to control in depth. If the dopth is set over an optical effect range, the optical element disturbs lens function.
However, if an fθ lens having an optical axis disposed at a position other than a center of an outer shape of a transfer surface extending in a principal scanning direction corresponding to a sang direction of beam is disposed to face a deflection section or deflector, or across the deflector, the fθ lens disposed at one side of the deflector and the other fθ lens disposed at the other side of the deflector must be disposed in an upward and downward inverted state to correspond positions of photoconductors each having four yellow, magenta, cyan and black colors to the transfer surface in coordinate.
Inverting one of fθ lenses to the other upwardly and downwardly causes deflection of the optical axis of one fθ lens from the optical axis of the other fθ lens.
Consequently, because a polygonal mirror having a large thickness, or a plurality of polygonal mirrors in the deflector are required, a load on a motor to rotate the polygonal mirror(s) increases, and therefore a large-scaled motor is needed.
Accordingly, in a light scanning device in which conventional plastic optical element are disposed to face each other in both sides of a light source section, there is a problem that relative deflection of the optical axes of the plastic optic elements occur, the polygonal mirror has a large size, the load of the motor to rotate the polygonal mirror increases, and the light scanning device and an image forming apparatus become a large size to lower the quality of an image to be formed.
Furthermore, the conventional plastic optical element has the same problem as in the above-mentioned light device.